Combined M-mode and two-dimensional echocardiography permits the following assessment of the genetics of hypertrophic cardiomyopathy: (1) no single mode of inheritance is typical of hypertrophic cardiomyopathy, although autosomal dominant transmission is most common; (2) a variety of phenotypic expressions occur which appear to have genetic as well as nongenetic etiologies, suggesting that hypertrophic cardiomyopathy may not be a single etiologically distinct disease entity; and (3) genetic counselling recommendations should be influenced by the particular pattern of inheritance demonstrated in each family.